1. Field of the Invention
The invention pertains to the field of keratoplasty and, more particularly, to thermokeratoplasty, especially electrically-induced thermokeratoplasty such as radio frequency or microwave thermokeratoplasty.
2. Description of the Related Art
The object of keratoplasty is to correct vision by reshaping of the cornea. For nearsighted persons, this reshaping involves flattening that ideally decreases the refractive power of the eye and causes the image to focus upon the retina, as opposed to focusing images in front of the retina. Invasive surgical procedures, such as laser-assisted in-situ keratonomileusis (LASIK) may be used, but there are associated complications, such as the dry eye syndrome associated with severing of corneal nerves, and the need for a healing period after surgery.
Thermokeratoplasty is a noninvasive procedure that may be used to correct the vision of near sighted persons by flattening the cornea. Generally, the cornea is heated to a point where collagen fibers in the cornea shrink, which results in stresses that tend to reshape the cornea. Thermokeratoplasty may be performed by the use or absorption of electrical energy, which is typically cycled in the microwave or radio frequency band for this purpose. Microwave thermokeratoplasty uses a near field microwave applicator to apply energy to the cornea, raising the corneal temperature. At about 60° C., the collagen fibers in the cornea shrink, and the onset of shrinkage is sudden. Resultant stresses from this shrinkage reshape the corneal surface. Application of energy in this manner may cause reshaping that flattens the central cornea when the energy is applied in circular or ring-shaped patterns around the pupil.
Devices and methodologies for microwave thermokeratoplasty are shown and described in U.S. Pat. No. 4,881,543 to Trembly et al., which is hereby incorporated by reference to the same extent as though fully replicated herein. The microwave applicator comprises an open-ended coaxial antenna driven at 915 MHz or 2540 MHz with an internal coolant system that drives flow of saline coolant transverse to the antenna axis. The '543 patent advances the art by providing applied electrical field theory for open-ended coaxial applicators and the related specific absorption rate, e.g., by using the Swicord and Davis technique in addition to heat transfer theory involving the Nusselt number, the Reynolds Number, and the dimensions of the gap between the antenna and the cornea.
Generally, these devices and methodologies are referred to as “microwave thermokeratoplasty” even though emissions at 915 MHz are slightly below the 1 GHz cutoff that many persons use to identify the microwave band. The term “radio frequency thermokeratonomy” may be used to describe energetic keratoplasty by excitation at lower frequencies. Microwave and radio frequency thermokeratoplasty may be used to achieve similar results, but the applied energy affects the tissue in different ways according to the various theories of operation where the radio frequency heating of tissue has a larger resistive heating component.
FIG. 1 is a side midsectional view illustrating a conventional microwave applicator 100 deployed for use in microwave thermokeratoplasty operations on a cornea 102. An oscillator drives microwave field emissions on concentric microwave conductors, such as conductive metal tubes 104 and 106. A dielectric 108 fills space between the tubes 104 and 106. A central passageway 110 permits the flow of coolant 112 into a flow gap 114 of about 0.3 mm to 0.5 mm between a terminal antenna end 116 and a Mylar™ (a trademark of E. I. du Pont de Nemours and Company Corporation, of Wilmington, Del.) film 118. The flow gap 114 places the central passageway 110 in fluidic communication with exit annulus 120 for circulation of coolant 112. The Mylar™ film 118 comes into direct contact with cornea 102 at interface 122, which may optionally have a concave down configuration adapted so as not to over-stress the cornea 102 by excessive flattening.
As shown in FIG. 1, the configuration of applicator 100 provides a microwave field that distributes itself downward through the coolant in flow gap 114 and into the cornea 102. The tubes 104 and 106 form an inefficient near-field applicator ideally having a penetration depth of less than about one millimeter, as opposed to a true antenna that can launch a wave. The small value of microwave penetration depth is an intentional design feature that is intended to protect the endothelium or back surface 124 of cornea 102 because the endothelium 124 is regarded to be incapable of regeneration after thermal damage. Coolant 112 flowing in the flow gap 114 cools the cornea at interface 122 by forced convection. The Mylar™ film 118 retains the coolant 112 in flow gap 114 and also prevents the flow of electrical current from tubes 104 and 106 into the cornea 102. By balancing the heating effects of applied microwave field against the cooling benefits of coolant 112, a local maximum of temperature is produced at near mid-depth of the cornea 102 while protecting the corneal epithelium (front surface) and the endothelium 124 (rear surface) from thermal damage.
A number of problems have arisen in use of prior microwave applicator devices. Chiefly, the amount of applied energy is unpredictably related in terms of a precise biological effect, such as by administering energy to produce a predetermined amount of vision correction. The thermal flux at depth in the cornea can be calculated according to theory with a high degree of precision; however, the thermal flux is not calibrated to a measurable biological effect in terms of an applied treatment modality. For example, it is undetermined what level of thermal flux is required to flatten a particular cornea to a desired level of diopter adjustment. This uncertainty is exacerbated by the characteristically sudden onset of thermally-induced shrinkage in the cornea. There is no clear way to determine in the course of treatment if, for example, the outermost layer of corneal cells known as the epithelium is undergoing thermal damage as a result of treatment, and this uncertainty can lead to a painful period of healing while the epithelium regenerates. Critical, small dimensions in the applicators may vary with machining errors, assembly or use, most notably in the dimensions of flow gap 114 for the coolant. Even small machining errors in these dimensions result in the applicator producing asymmetric treatment rings and associated astigmatic effects on the “corrected” vision resulting from use of these devices.
There is a need to improve the predictability of effectiveness of microwave thermokeratoplasty applicators and to reduce the unintended harm that such devices may produce.